Apparatus for generating shock waves

ABSTRACT

A device for generating shock waves, specifically for medical applications, has electrodes arranged in a liquid medium, a high voltage being applied to the electrodes to generate an electrical breakdown. Particles added in powder form are suspended in the liquid medium. In order to maintain the particles in suspension, a thixotropic additive is added to the liquid medium.

[0001] The invention relates to a device for generating shock waves, specifically for medical applications, according to the preamble of claim 1.

[0002] Shock waves are increasingly employed in medicine, for example for breaking up concretions in the body, for stimulating bone formation, for treating orthopedic diseases, and for combating pain. To generate shock waves, devices are often used in which two electrodes are arranged a certain distance apart in a liquid medium. A high electric voltage is applied to the electrodes until an electrical breakdown results within the liquid medium. The heat developed from the electrical breakdown causes the liquid medium to vaporize, thereby creating the pressure wave.

[0003] The action of triggering the spark discharge causing the electrical breakdown depends on the high voltage applied and the distance between the electrodes. To ensure reliable triggering of the spark discharge even at the critical distance between the electrodes, the method of adding conducting, semiconducting, or polarizable particles in powder form to the liquid aqueous medium has proven advantageous (EP 0 781 447 B1). The gas formed during the spark discharge may impair the formation of subsequent spark discharges, and specifically impede the propagation of the shock wave. A known remedy for this (DE 197 18 451 A1) is to add to the liquid medium a powdered catalyst which reduces the gas formation and promotes the recombination of the gases formed. Both the particles promoting the spark discharge and the particles of the catalyst are added in powdered form and suspended in the liquid medium.

[0004] To ensure that the particles have the desired effect, they must be suspended specifically in the vicinity of the electrodes. Counteracting this suspension is gravity which, over time, causes the particles increasingly to settle on the bottom of the container holding the liquid medium. A lowering of the concentration of the particles promoting discharge in the vicinity of the electrodes may impair or completely impede the triggering of the spark discharge. A lowering of the concentration of the suspended catalyst particles in the vicinity of the electrodes may result in premature formation of gas, which will delay the formation of the spark discharge and reduce the generation of pressure.

[0005] In order to counteract this disadvantageous settling of the suspended particles, the volume of liquid surrounding the electrodes may be restricted by adding a hood. Although the particles here do settle as well due to gravity, they are agitated by each discharge and again dispersed in suspension. Several discharges may be required, however, until the optimal suspension for operating the shock-wave source is reached. In addition, the initial voltage employed must be higher in order to achieve this first electrical breakdown. Only after the particles promoting the breakdown are sufficiently suspended can the voltage be returned to its normal operating level.

[0006] Another approach is to store the powder particles in an ancillary container which is introduced into the volume of the liquid medium. Upon each discharge, particles are released from this container by the resulting pressure wave and enter suspension.

[0007] Both approaches have the disadvantage that additional components (hood, storage container) are required which must be introduced into the volume of the liquid medium. In addition to the extra design and materials expense, these components disturb the propagation of the generated shock wave. Additionally, the shock-wave source does not immediately achieve the optimum properties at start-up.

[0008] The goal of the invention is therefore to provide a device for generating shock waves in which the particles suspended in the liquid medium may be retained in the vicinity of the electrodes without the extra design and materials expense and without impeding the generation of the shock waves.

[0009] This goal is achieved according to the invention by a device having the characteristic features of claim 1.

[0010] Advantageous embodiments of the invention are provided in the subclaims.

[0011] According to the invention, a substance is added to the liquid medium in which the electrical breakdown for generating the shock waves occurs, which substance imparts to the liquid medium a structural viscosity with a thixotropic index greater than 1. Structural viscosity here means that the viscosity of the liquid medium decreases as the velocity gradient within the liquid increases. This means that the viscosity falls when the liquid medium is moved. When the velocity gradient in the liquid medium decreases, i.e., when the liquid medium comes to rest, the viscosity again rises significantly due to thixotropy. The thixotropic index here is a measure of the structural viscosity and is defined as the ratio of the viscosity at two different velocity gradients, specifically the ratio of the viscosity at one velocity gradient relative to a velocity gradient 10 times greater.

[0012] This thixotropy property of the liquid medium is exploited according to the invention to maintain the particles in suspension in the liquid medium over a longer period of time and impede their settling to the bottom under the influence of gravity. When the device is not in operation, the liquid medium is at rest and has a high viscosity. This high viscosity maintains the added particles in uniform suspension for an extended period of time such that the device remains in a state ready to operate and with full functional performance even after an extended period of non-use without additional measures having been taken. The liquid medium is moved by the electric discharge during formation of the shock waves such that the viscosity drops significantly due to the thixotropy property. The propagation of pressure waves within the liquid medium is thus not impeded by the thixotropic additive.

[0013] The structural viscosity of the liquid medium is advantageously obtained by adding an inorganic silicon compound, specifically silicic acid. The preferred additive is a highly dispersed silicic acid.

[0014] The highly dispersed silicic acid does not significantly affect the electrical properties of the liquid medium. The conductivity of the liquid medium, the latency period required for the formation of the electrical breakdown, and the quantity of the charge flowing from the breakdown all remain unaffected by the added silicic acid. In addition, silicic acid is heat-resistant and chemically stable—with the result that the properties of the structural viscosity are not destroyed by the plasma created during breakdown. The metal alloy of the electrodes is also not corroded by the added silicic acid.

[0015] The highly dispersed silicic acid added in the form of amorphous silicon dioxide particles forms a three-dimensional network in the liquid medium through agglomeration, with the network producing the high viscosity in the quiescent state. This also has the additional advantage that the particles suspended in the liquid medium are kept a certain distance apart by the network of highly dispersed silicic acid, thereby preventing the suspended particles from conglomerating and, specifically, preventing the surface area important to the catalytic action from being reduced.

[0016] The following example illustrates the mode of action of the invention: A catalyst in the form of 0.4 g of palladium on activated charcoal is added to a volume of approximately 18 ml of liquid. The addition of 0.2 g of highly dispersed silicic acid (HDK V15 from Wacker Chemie GmbH) results in the sedimentation rate of the suspended catalyst being reduced by a factor of at least 10 to 20. Even after a standing time of more than 10 minutes, the shock-wave source is triggered immediately at the lowest power level. Without the addition of the silicic acid, the shock-wave source is no longer triggered and must be “primed” at considerably higher high-voltage levels.

[0017]FIG. 1 is a simplified pictorial illustration of a container that includes the liquid, the catalyst, and the additive.

[0018] Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions, and additions to the form and detail thereof may be made therein, without departing from the spirit and scope of the invention. 

1. Device for generating shock waves, specifically for medical applications, including: a liquid medium, electrodes located in the liquid medium to which a high voltage is applied to generate an electrical breakdown, and particles added in powder form to the liquid medium, characterized in that the liquid medium has a structural viscosity with a thixotropic index greater than
 1. 2. Device according to claim 1, characterized in that the structural viscosity is produced by a thixotropic additive.
 3. Device according to claim 2, characterized in that the additive is an inorganic silicon compound.
 4. Device according to claim 3, characterized in that the additive is silicic acid.
 5. Device according to claim 4, characterized in that the additive is a highly dispersed silicic acid. 